This invention relates to a tape drive and control means for sound motion picture systems, and more particularly, to a system for reducing flutter in sound reproduction from a magnetic tape.
For sound accompaniment to motion picture systems, it has been found useful to interleave a magnetic tape with the photographic film so that synchronization can be achieved between the sequential frames of a scene and audio information associated with the scene. In such case, the film drive motor of the system is designed to unspool the interleaved film and tape from a supply reel. Film and tape are directed along separate paths before combining them in interleaved fashion on a takeup reel. A separate capstan motor is employed for driving a capstan engaged with the tape whereby the latter is moved past a transducer which reproduces the sound recorded on the tape.
In motion picture systems, good fidelity in the reproduced sound requires isolation of the intermittent motion of the film from the continuous movement of the tape required for reproducing sound recorded thereon. Such isolation is achieved by creating and maintaining a loop in the magnetic tape of predetermined minimum size. In conventional systems, the rate of the film drive motor is generally varied to maintain this loop. However, the provision of a sound system as an accessory to an existing projection system generally requires that the speed of the sound capstan be varied rather than the film drive. The control system for the capstan, therefore, must relate the speed of the capstan motor to the speed of the film motor such that the loop is maintained within selected limits at all times. Where the sound system is an accessory, this requirement for maintaining the size of the loop is difficult to meet since the film drive motor is not accessible and each motor must operate independently of the other.
In addition to there being a need to establish and maintain a loop in the magnetic tape, short duration accelerations in the tape movement must be minimized to prevent introduction of flutter (i.e., easily noticed and undesirable pitch changes) in the reproduced audio signal.
U.S. Pat. Nos. 3,832,045; 3,838,447 and 3,850,513 describe flutter control systems in which sample portions of information recorded with a reference signal are read from the tape into a memory at a rate determined by a reference signal and then read from the memory at a fixed rate to thereby reduce flutter. Optionally, a speed control for the tape drive is employed to eliminate low frequency errors. While the abovenoted flutter control systems operate satisfactorily in many sound systems, they have no provision for maintaining an adequate tape loop independently of slow variations in the speed of the film drive motor.
It is therefore an object of the present invention to provide a new and improved flutter control system for sound reproduction, and an improved sound motion picture system wherein the control system for the capstan motor changes its speed in such a way as to minimize flutter while at the same time maintaining the necessary loop in the tape independently of the film drive motor.